In digital microscopy illumination-dependent, systematic aberrations occur which are shaped with different strengths in particular when using exchangeable objectives and motorized zoom systems from device to device and according to the actual zoom setting.
Such illumination-dependent, systematic aberrations can be, for example, reflexes in a coaxial reflected light illumination which are problematic and/or make the use of coaxial reflected light illumination impossible, in particular in the case of objectives with a comparatively large focal length.
Since the illumination types are frequently combined with each other in microscopy, these aberrations can absolutely also occur combined in different sizes.
Various methods are known for the image correction in optical systems, in particular from digital camera technology. For example, in the case of photo objectives vignetting is used as a correction means for image errors (Koma).
There are furthermore attempts for the digital correction of multiple reflexes in photo objectives. In order to correct artefacts in the taking of images, the method of dark image reproduction is customary.
An image recording system from the application area of “machine vision” is known from US 2002/0041383 A1. In this system all geometric distortions by camera, objective, illumination and computer should be measured and digitally corrected in order to calculate a distortion-free digital image in the correct enlargement. The images are corrected and are all scaled to the same enlargement. A simultaneous correction of different aberrations takes place. A system calibration takes place with a special geometric and/or illumination calibration mask, for which two cameras are required. The described method is extremely expensive.
When using a coaxial reflected light illumination with a microscope, light is coupled into the observation beam path by a beam divider. Before the light strikes the object it passes through at least one optically active element of the observation lens. Certain parts of the light are reflected on the boundary surface of such an element, in particular in the vicinity of the optical axis, and are mixed with the useful light propagated from the object by the observation lens. This reflected light forms the so-called top illumination reflex.
In modular zoom systems such top illumination reflexes can occur in certain objectives which change their shape, color and intensity during zooming and also during the change to another objective with the same construction.